Uhlenbeck on the Ann Arbor Summer Symposia

Since my involvement[s] with the theory of β-decay were inspired by the Ann Arbor summer schools in theoretical physics, I thought it might be of interest, by way of introduction to the Konopinski recollections, to reminisce a little about some of these summer sessions. I think that the Ann Arbor summer schools really deserve a study by a historian of science. In their hey-days (from the mid twenties to the beginning of the war) almost all the leading theorists lectured or visited Ann Arbor some time or other during eight weeks in the summer. And because of the small size (±50 participants), the informality and the often vivid and sharp discussions, they had, I believe, a strong influence on the development of physics, and they surely influenced me! Often in the fall I continued to wak[sic] to try to digest what I had learned in the summer!
First let me tell something about the 1930 summer school when Fermi and Ehrenfest were the main lecturers. Fermi gave a course on the quantum theory of radiation, from which I learned properly for the first time the theory of quantized fields. I helped with the publication in the Rev. Modern Physics (Jan ’32), and if I may say so I still think these lectures are the best introduction to quantum electrodynamics which are available.
In 1931 the main lecturers were Kramers and Pauli, and for a shorter period Sommerfeld and Oppenheimer. It was a bit of wild summer (Pauli broke his arm!) but let me not elaborate on that. I remember very clearly a lecture Pauli gave on the famous energy crisis in β-decay. Pauli was not always a good lecturer, but if he had something to say and if was well prepared, then he was splendid. He explained the experiments of Ellis and Wooster and of Lise Meitner, which showed conclusively that the continuous β-spectrum of Ra E could not be of secondary origin (produced by the energy losses of the electrons. Nuclei in the same state emitted electrons of different energies and ended up again in the same state, as judged by their further decay history. This energy puzzle of 1922 had been dormant for many years, but in the early thirties (when nuclear physics became the “frontier”!) it plagued the “radioaktive Damen und Herren” (as Pauli called them in a letter of Dec. 1930, see his reminiscences written in 1956, Coll. Papers II p. 1313). Pauli then explained the alternative: either one should prepare oneself with Bohr to give up the energy principle for the individual process, or one assumed that with every electron, a very light and neutral particle is emitted, which carries away the remaining energy and which is not caught in the calorimeters of Ellis and Meitner. Both alternatives were revolutionary, but Pauli preferred the second as the least of two evils! It is remarkable that Pauli did not elaborate his idea further. As he tells the story, he talked with Fermi about it in the fall of 1931 in Rome. Fermi was interested and baptized the neutral particle the neutrino, but again, no formal theory was attempted at that time. The controversy Bohr vs. Pauli continued and was further discussed at the Solvay Congress of October 1933, where also Fermi was present.
I believe that only after this congress Fermi started to put the basic idea that β-decay is represented by the reaction: N → P + e- + ν and positron-decay by P → N + e+ + ν’, in a mathematical form. In 1932, when I was with Fermi in Rome, and at the summer school of 1933, where he and Bohr were the main lecturers, the β-decay puzzle was hardly mentioned. Fermi and I worked hard on the positron theory (which was then the “frontier”!), and Bohr still defended his view. He gave up only in 1936!
I received Fermi’s first paper (“Tentativo di una teoria dell’emissione dei raggi β”, Ricerca Scientifica, Vol. 2, Dec. 1933) early in ’34. Because of the analogy with the quantum theory of radiation (which I had learned from Fermi) and because I had become well versed in the relativistic wave equation through the work on the so-called spinor analysis which I had done with Otto Laporte, I could appreciate Fermi’s paper right away and I became very enthousiastic[sic]. Together with Huge Wolfe I applied Fermi’s theory to the discussion of the stability of the free neutron, and then after the second paper of Fermi (25./. Phys. 88, 161, 1934) Konopinski and I started to analyse the theory in detail. Several points became clear to us (as the arbitrariness in the coupling “Ansatz” and the association of neutrino or anti-neutrino to the two basic reactions), but the main point which disturbed us was that the predicted shape of the β-spectrum did not seem to agree with experiment. Konopinski found that if the basic statistical or phase-space factor:
p2e p2ν dpe dpν
in the Fermi theory was changed to:
p2e p4ν dpe dpν
then the agreement with experiment became much better. This lead to the so-called K-U coupling “Ansatz” which involved the derivative of the neutrino wavefunction, and which received quite some attention for a while. It disappeared when must better experiments were made in the forties.
Finally one should say that in the thirties the β-decay theory was not a central problem since it did not lead to anything. It is, I think, significant that Fermi never came back to it. As a curiosum, let me mention that for awhile I hoped that the theory would explain the nuclear forces. The Fermi interaction does lead to a force between proton and neutron, which is very weak, but thank God divergent! So one could hope that with a cut-off, and then summing the results to all orders of perturbation and finally let the cut off go to infinity, a strong force might come out. I worked on this with Kahn when I was in Utrecht, but it was clearly desperation physics and nothing came out.
Only later when the β-decay was recognized as one of the weak interactions more general points of view were developed. You will hear more of that from Konopinski.

Leave a comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: